Datasheet

A5973D Application information
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In short-circuit conditions V
OUT
is negligible so during T
OFF
the voltage across the inductor
is very small as equal to the voltage drop across parasitic components (typically the DCR of
the inductor and the V
FW
of the free wheeling diode) while during T
ON
the voltage applied
the inductor is instead maximized as approximately equal to V
IN
.
So the Equation 28 and the Equation 29 in overcurrent conditions can be simplified to:
Equation 30
considering T
ON
that has been already reduced to its minimum.
Equation 31
considering that f
SW
has been already reduced to one third of the nominal.
In case a short circuit at the output is applied and V
IN
= 12 V the inductor current is
controlled in most of the applications (see Figure 15). When the application must sustain the
short-circuit condition for an extended period, the external components (mainly the inductor
and diode) must be selected based on this value.
In case the V
IN
is very high, it could occur that the ripple current during T
OFF
(Equation 31)
does not compensate the current increase during T
ON
(Equation 30). The Figure 17 shows
an example of a power up phase with V
IN
= V
IN MAX
= 36 V where Δ
IL TON
> Δ
IL TOFF
so the
current escalates and the balance between Equation 30 and Equation 31 occurs at a current
slightly higher than the current limit. This must be taken into account in particular to avoid
the risk of an abrupt inductor saturation.
Figure 15. Short-circuit current V
IN
= 12 V
I
L TON
Δ
V
IN
DCR
L
R
DSON
+()I⋅–
L
----------------------------------------------------------------
T
ON MIN
()
V
IN
L
---------
250ns()≅=
I
L TOFF
Δ
V
D
V
out
DCR
L
I•++()–
L
---------------------------------------------------------------
3T⋅
SW
()
V
D
V
out
DCR
L
I•++()–
L
---------------------------------------------------------------
12μs()≅=